1. Field of the Invention
This invention relates to the editing and publishing of video data; and, in particular, relates to the design of systems using image data compression and decompression techniques for editing and publishing of video data.
2. Discussion of the Related Art
2.1 Compression
In the present state of the art, digital video data are highly compressed to minimize the requirement for storage resources. To achieve a high compression ratio, both interframe and intraframe compression algorithms are typically applied to a video sequence. Intraframe compression is achieved by minimizing information redundancy within a frame, and sacrificing some image definition. By contrast, interframe compression removes temporal redundancy in the video sequence. Various standards are accepted today using both intraframe and interframe techniques for video data compression. One such standard is the MPEG (Motion Picture Experts Group) standard. Within the MPEG standard intraframe compression is applied to a selected number of frames ("I-frame") in a video sequence. Using the I-frames as references, interframe compression within the video sequence is accomplished by encoding the incremental differences between frames. Such incrementally encoded frames are called "P-frames or predicted frames", when the incremental difference encoded in each frame relates to either a previous or future frame. Alternatively, when the incremental difference encoded in each frame relates to both a future and a previous frame, such frames are called "B-frames or bidirectional frames". P-frames can also serve as bases for encoding other P frames and B-frames.
Another standard that is also widely adopted is the JPEG (Joint Photographic Experts Group) standard, which does not provide for interframe compression. Naturally, JPEG does not achieve the high compression ratio achieved under the MPEG standard. Typically, for the same quality in the resulting image, MPEG interframe compression achieves several times the compression ratio achieved in a JPEG compression.
2.2 Video Editing
Commercial video sequences, such as motion pictures, advertising, or music videos, are heavily edited after the images are captured. Typical editing activities include special effects, titling, mixing, fades and wipes. At present, a video sequence is usually first captured on tape in "analog" form, which is then digitized for editing. Editing can occur in a large post-production facility with special equipment to perform each of the various functions, such as mixing, titling, and other special effects. Currently, to preserve the quality of the video sequence, video editing occur at the full definition of the digitized images (i.e. no compression). When editing is completed the video sequence is once again converted to analog form on a "master" tape, which is then duplicated for publishing.
The cost of the video editing procedures described above is expensive primarily because of the high cost of the full-definition editing equipment. Typically, customers of a post-production facility are charged for the time they are "on-line," i.e. the time during which the video editing equipment is used. In the future, it is believed that the use of both video compression techniques and the increasing power of the personal computer can lead to lower cost of video editing.
Video editing requires random access to frames. Because of this requirement, the interrelated nature of frames under an interframe compression scheme renders the use of interframe compressed video as input data to video editing impractical. Thus, existing video editing systems use only intraframe compressions to compress frames. Recently, JPEG-based video editing software programs and hardware accelerators are available on personal computers to mimic the activities in the post-production facility. Such programs, although not providing the image definition necessary for production quality editing, provide a mock edit session "off-line." The result of such a mock edit session is a machine-readable "edit decision list" file, which can be used to automatically direct an edit session, using the production quality equipment at the post-production facility. In this manner, on-line time, hence cost of video editing, is significantly reduced.
1.3 Digital Video Publishing
As mentioned above, video sequences are usually published today on a tape. Recently advances in image processing integrated circuits provide MPEG decoders to decode high quality digital video sequences at relatively low bit-rates, which can be stored on a compact disk ("CD"), distributed on a phone line, or delivered over a local area network. For digital video distribution ("video publishing"), the edited video sequence, which is stored today on a master tape, must be redigitized and compressed under an interframe compression standard. There is a need for a system available on the desktop to provide both video editing and publishing.